Why am i getting wow lag?

I have high fps and great ping or ms, so im trying to figure out the problem. I already tried messing around with the v sync, trip buffering, and reduce input lag. I have an old macbook ( 1.83Ghz, intel core duo) with the 10.5.8 update. Sometimes i play fine, but a lot of the time when i do a cast, for example heal, my guy just stand there still casting until i move ( screen lag). All my settings are on low and cant be changed higher, so i guessing its the video card in my mac.